This interdisciplinary project has as its objective the elucidation of the biochemical mechanism by which aminoglycosidic antibiotics (neomycin, streptomycin and related compounds) cause permanent hearing loss. It begins with the hypothesis that these antibiotics bind to polyphosphoinositide lipids and displace calcium disturbing calcium homeostasis and cell membrane function. The hypothesis will be investigated by a combined in vivo/in vitro approach. In vivo, aminoglycosidic antibiotics, fragments and derivatives will be tested by cochlear perfusion in the guinea pig for their effect on cochlear microphonic potentials and phospholipid labeling by 32P-orthophosphate. In vitro experiments will be conducted with synaptosomes from brain to study the influence of these drugs on enzymes and substrates of phosphoinositide metabolism. This in vitro system will also be investigated for its potential usefulness as an in vitro correlate of in vivo toxicity. Studies with monomolecular films of phospholipids will focus on drug/calcium interactions, on the stoichiometry of the drug/lipid complex and on the determination of those sites on the antibiotic which are involved in its ototoxic action.